Monitoring of performance and physiological parameters is of importance for fitness and athletic training. The fitness industry has for long proposed devices for performance monitoring that are able to measure the distance, compute average/maximum speed.
Modern training computers (personal trainers) further determine the heart beat rate and incorporate training programs, and optionally integrate a GPS sensor for more precision in the performance monitoring. For example, the heart beat rate measurement is used in fitness and athletic training for determining working capacity or stress level.
Usual training protocols are based on zones related to percents of maximum heart rate (HRmax—the highest number of heart beats per minute an individual can reasonably achieve in a stress exercise). Training between 70% and 90% of HRmax is particularly important for improving the ability to sustain high exercise intensities. Such work rates tend to correspond to an important physiological change: the energy production pathway turns from an oxygen consumption (aerobic) mode to an anaerobic mode. That transition is called the anaerobic threshold (AT). Exercising at or above the transition is not possible for long since lactic acid accumulates in the tissues, leading to muscle pains and breath shortages. Equivalent terminology for the anaerobic threshold is either the lactate threshold or the ventilatory threshold, as a sudden rise of both lactic acid in blood and ventilation occurs at that point. Ventilation (or minute ventilation) is scientifically defined as the product of the breathing frequency and the volume of inhaled/expired air.
The anaerobic threshold is nowadays determined in special test laboratories by means of standardized protocols. The most classic way of assessing the AT is by assessing blood lactate continuously throughout the test and identifying the point where blood lactate begins to accumulate (also known as lactate threshold).
Another conventional method for determining the anaerobic threshold is the well known V-slope method developed by Karlman Wasserman, based on the evolution of the ratio of oxygen uptake (VO2) to carbon dioxide output (VCO2). Before the anaerobic threshold is reached, VO2 and VCO2 tend to rise at roughly the same rate, whereby a best-fit line through these points is close to 1. Once the AT is reached, VCO2 will rise faster and the line fitting through the points corresponding to the anaerobic region will have a slope greater than 1. The intersection point between the two lines is thus the threshold between the aerobic and anaerobic regions. Since the threshold is determined from the ventilatory response, it is generally referred to as ventilatory threshold (VT).
Today, the VT or AT is a parameter that is primarily used in training endurance athletes to determine suitable training capacities, respectively monitor their training capacities.
When the heart beat rate value corresponding to the VT is known, a desired training capacity can be accurately maintained by means of continuous heart beat rate monitoring. However, as explained above, the presently available methods for determining the AT or VT values are based on difficult lactic acid or breathing gas measurements, requiring taking of blood samples and expensive laboratory equipment and staff; in other words: using invasive and non-portable equipment.
It is thus desirable to have portable equipment that allows field measurement of the anaerobic threshold, so that athletes, sportsmen or fitness enthusiasts can monitor their VT during real-life and real-time exercise.
U.S. Pat. No. 5,810,722 assigned to the Polar Electro company describes a device for assessing a person's VT under a gradually increasing stress. The respiratory frequency and volume are calculated on the basis of the ECG signals to exploit a respiration frequency vs. heart rate graph, or a ventilation vs heart rate graph, where the VT appears as a break point. A difficulty related to this method is that it is entirely based on ECG signals. Indeed, determining the respiratory response from the ECG, although theoretically possible, requires a high quality signal, which may not always be compatible with field measurements.
DE 102 48 500 describes a method for determining the AT of a subject during exercise that can be implemented by a portable system. The breathing frequency is determined by means of an expandable belt encircling the sportsman's chest and including a strain gauge. The AT is detected as an increase in the breathing frequency, typically by comparing a current breathing frequency to a previously determined breathing frequency.
While the system described in DE 102 48 500 may be appealing in that it provides an in field VT sensor, which can be easily implemented, its scientific rationale has been criticized by some authors. For example, Cottin F. et al. in “Ventilatory Thresholds Assessment from heart rate variability during an incremental exhaustive running test”, Int J Sports Med, 2006, ISSN 0172-4622, conclude that assessing the VT from the breathing frequency is not possible during a running test.
As a matter of fact, the determination of physiological parameters during the actual performance of physical activity in real-life is a challenge. A first difficulty is the reduced number of signals, since blood taking and classic laboratory equipment (spirometers etc.) cannot be used. A further difficulty is the quality of the signals measured with portable equipment. Obviously, this reduces the possible methods usable to determine the VT. Thirdly, it is desirable to be able to determine physiological parameter(s) such as VT during any kind of training or performance, and not only for pre-defined effort tests.
In this connection, it may be noticed that although a variety of papers discuss and compare methods for determining the VT, they typically rely on finished data sets obtained from a known population of athletes all performing the same pre-defined tests (see references 1-3). Hence, the classical approach followed in the literature consists in acquiring, during the performance of a standardized test, the experimental data for the test population, and then applying several methods to the experimental data, such as e.g.: respiratory exchange ratio, V-Slope, Ventilatory equivalent for O2 . . . . Authors then have typically discussed the verifiability, repeatability and/or sensitivity of such methods, relying on statistics (e.g. mean and standard deviation).